This invention relates generally to surface treatment methods for single crystalline and polycrystalline silicon surfaces and more specifically to argon-plasma treatments and chemical-vapor deposition methods for such surfaces in semiconductor devices.
Field-effect transistors may be provided with a polysilicon gate region having a layer of metal placed thereon to lower the sheet resistance of the polysilicon gate region from 20 ohms per square to as little as 2 ohms per square to improve the speed of the gate region. This layer of metal acts as a shunt to produce an increase in circuit speed. In other applications, it is desired to place a thin layer of refractory metal on a single crystalline surface of a silicon diode to form either a Schottky-barrier diode or an ohmic contact, depending on the type and doping concentration of the semiconductor.
In the past, molybdenum and tungsten have been investigated for use as self-aligned refractory metal gates in large-scale integrated-circuit (LSI) technology. More recent developmental efforts in LSI have produced an increasing interest in polysilicon gates and a corresponding declining interest in refractory metal gates. However, in very-large-scale integrated-circuit (VLSI) technology, doped polysilicon which is typically used as the gate material has a sheet resistance of 20-50 ohms per square for a 0.5 micron thick film; even with a 10 ohms per square film, the RC time delay of fine polysilicon lines outweighs the speed advantages gained from short channel transistors. (RC time delay is a delay in the circuitry due to the resistance and capacitance in the circuit.) Therefore, as line widths in VLSI circuits shrink, the major speed limitations arise from the RC time constant associated with silicon gates and polysilicon interconnect lines and not from transient time between source and drain of a field-effect transistor. To reduce interconnect resistivity, it is desirable to deposit refractory metals or metal silicides on top of the polysilicon lines.
Refractory metals for VLSI applications are customarily deposited by three different methods: sputtering, evaporation, and chemical-vapor deposition. Each approach has advantages and disadvantages. The main advantage of the sputtering process is that it is possible to sputter almost any material. Both pure refractory metals and refractory metal silicides can be sputtered. However, sputtering machines are complicated and require considerable maintenance. Also, elaborate etching techniques have to be employed to obtain discontinuity-free etching profiles of a silicide/polysilicon composite.
Evaporation of refractory metals has been investigated as a means for forming VLSI. However, evaporation has many of the deficiencies associated with sputtering. For example coverage is poor and the deposition process is complex using evaporation techniques.
Chemical-vapor deposition (CVD) and low-pressure chemical-vapor deposition (LPCVD) of refractory metals offer several advantages over sputtering and evaporation techniques. CVD of refractory metals can provide good coverage, reduced system complexity, and higher purity deposits. In addition, CVD is an attractive alternative to the above methods because it does not need an additional photolithography step when the chemical-vapor deposition process is a selective process in which the refractory metal is deposited only on the polysilicon gate area and not on surrounding silicon-dioxide areas.
Tungsten films have been formed in the past by hydrogen reduction of tungsten hexafluoride according to the equation EQU 3H.sub.2 +WF.sub.6 .fwdarw.W+6HF. (1)
This reaction has been discussed in great detail in Morosanu, C-E. and Soltuz, V, "Kinetics and Properties of Chemically Vapour-Deposited Tungsten Films on Silicon Substrates" Thin Solid Films, 52(1978) 181-194. This article is incorporated herein by reference. As can be seen from equation (1), hydrofluoric acid is a byproduct of the hydrogen reduction of tungsten hexafluoride. This is an undesirable byproduct because HF etches any silicon oxide typically surrounding the gate region. Also, the thickness of the film produced by this method is difficult to reproduce accurately even under identical deposition conditions. Also, under certain flow-rate conditions, tungsten will deposit on silicon dioxide or other surrounding materials. Thus, the deposition process is not a totally selective deposition process. This is true particularly in instances in which it is desired to produce a film of tungsten having a thickness greater than 2,000 angstroms.